A cellular communication system is one in which mobile stations, such as portable or vehicle carried telephones or radios, wireless enabled computing devices, personal digital assistants and the like, can communicate via a network infrastructure. The infrastructure generally includes a plurality of base stations (base transceiver stations) which may be fixed in position and which may form a communication network. Each base station has one or more transceivers which serve mobile stations in a given region or area, known as a ‘cell’ or ‘site’, by wireless communication. The cells of neighbouring base stations are often overlapping.
Communications to and from mobile stations include system control communications and traffic communications. System control communications are made between the mobile stations and their serving base stations to establish and maintain operation in the system in accordance with a pre-determined operating protocol. Traffic communications are made between a mobile station and another terminal to transmit user information such as speech or data, e.g. numerical or text data, picture information or video information. A traffic communication between mobile stations which are currently in the same cell may be established by a single base station or even directly between the mobile stations without the involvement of a base station. Traffic communications between mobile stations which are currently in different cells may be established by involvement of a plurality of base stations serving the various cells.
In the latter situation, a calling mobile station making a traffic communication or ‘call’ to a target mobile station makes a wireless transmission initially to a first base station serving the calling mobile station. Such a transmission from the mobile station to its serving base station is known as an ‘uplink’ transmission. The transmission received by the first base station is routed by the first base station to a second base station serving the target mobile station. In the case of a group call from the first mobile station to a plurality or ‘group’ of further mobile stations, the transmission is routed by the first base station to each of a plurality of receiving base stations serving the target mobile stations. The transmission from the first base station to the receiving base stations may possibly take place via other network nodes such as one or more routers or other base stations. The transmission is routed by each receiving base station to the target mobile station or stations served by that base station. Such a transmission from a base station to a served mobile station is known as a ‘downlink’ transmission.
Another possibility is for a traffic communication to be established between a mobile station and a target terminal other than another mobile station, such as a fixed terminal operated by a system operator or a fixed terminal connected to a recording device for recording the content of communications sent or received by mobile stations. In such a communication, the mobile station may be a transmitting or receiving terminal. In this case, the first base station serving the mobile station routes an uplink transmission received from the mobile station to the other terminal and routes a transmission from the other terminal as a downlink transmission to the target mobile station.
Thus, downlink traffic transmissions from a base station to one or more target mobile stations or other terminals served by or linked to the base station can originate from various remote transmitting sources. There can be a delay, a ‘transmission delay’, between transmission of a traffic signal by a transmitting source and reception by a base station which has to forward the signal (to a served target mobile station or other terminal). There can also be a delay, a ‘BS processing delay’, in the receiving base station between reception of the traffic signal and forwarding of the signal as a downlink wireless signal.
In general, the transmission delay depends on the location of the transmitting source and the route of the transmission to reach the serving base station of the target mobile station(s).
In order for a base station to send the downlink traffic signal to a target mobile station, a downlink traffic channel has to be allocated to allow the signal transmission to take place. The allocation of the downlink traffic channel may be made by a processor such as a resource scheduler associated with the base station, e.g. incorporated within the base station. The form of the allocated channel depends on the communication protocol operated in the mobile communication system. For example, some systems such as a TETRA (Terrestrial Trunked Radio) system, that is a system operating according to the protocol specified in the TETRA standard as defined by ETSI (the European Telecommunications Standards Institute), use a time slotted Time Division Multiple Access (TDMA) protocol in which traffic signals are sent in allocated time slots on different allocated carrier frequencies. In a TETRA system, the time slots have a length of 14.16667 milliseconds, four such slots make up a frame and eighteen frames make up a multiframe having a length of approximately one second.
Where a downlink traffic signal transmission is to be made by a base station there can be an alignment delay (which is part of the BS processing delay mentioned above) between the base station having the downlink traffic signal ready to be sent and a traffic channel being available for the signal transmission to be made. Undesirably, this alignment delay can have a length or duration which is significant. For example, in a TETRA system, the alignment delay can have a length of up to one frame. Such a delay can seriously affect the quality of the received information at the target mobile station or other terminal, e.g. the quality of received speech information at the beginning of a speech conversation. This problem is noticeable especially in a full duplex system, that is a system in which uplink transmissions received by a base station and downlink transmissions sent by the base station can be made at the same time, e.g. in the same time slots on different carrier frequencies.
Thus, there exists a need for a base station and an associated processor, and a system and a method employing the base station and the processor, for use in mobile communications, which addresses at least some of the shortcomings of past and present techniques and/or procedures for making downlink transmissions.
Skilled artisans will appreciate that items shown in the drawings are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the items may be exaggerated relative to other items to assist understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood items that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.